Photoelectron emission enhancement mechanisms in metals and semiconductors have been proposed involving the creation of color centers in thin coatings of CsBr films by UV radiation damage. Here, the creation of color centers refers to energy states inside the gap that align with the Fermi level of the substrate. They are created to allow electron transitions to the conduction band with photon energy less than the gap energy. The states created with the 4.8 eV radiation have a relatively narrow width and have an energy of about 3.8 eV inside the gap. In addition, other proposed possible color center mechanisms allow Br atoms to move to the CsBr vacuum surface. It is postulated that Br neutral atoms are expelled to the vacuum leaving a charged Cs layer, which lowers the work function of the photocathode structure. This motion of Br atoms away from the CsBr film if it occurs may be consider as ablation limiting the lifetime of the photocathode. However, only a monolayer of atoms is required to lower the work function of the CsBr/vacuum interface, and the CsBr films may be hundreds of monolayers thick. Similar atomic motion may occur to form a Cs layer at the CsBr/substrate interface lowering the work function to electrons directly emitted by the substrate metal or other material and transmitted by the CsBr film. Typical operation of a CsBr/metal photocathode shows an initial increase in the photoelectron yield reaching a maximum and then decays to reach a steady state value. This behavior is attributed to the formation of a Cs layer on the vacuum CsBr interface surface reaching equilibrium with contaminants (mainly C and O) in the vacuum system. Successful operation for hundreds of hours with a laser spot of about 1.5 microns has been obtained at a vacuum pressure of 1×10−9 torr. Operation for thousands of hours is possible by locating the laser spot on fresh unexposed areas of the photocathode in a sequential manner.
It has been known for some time that alkali halides develop color centers when subjected to UV or low energy e-beam irradiation. For the UV case, it was discovered that CsBr films (1-25 nm thick) deposited on metal or semiconductor layers can increase the photoelectron yield of the underlying substrate by a large factor when illuminated with UV radiation with a photon energy less than the CsBr bandgap of about 7 eV. The use of CsBr based photoelectron sources for electron beam lithography and related applications has been hampered by the need for bulky and expensive UV lasers to provide the short wavelengths (e.g. 257 nm) necessary to generate sufficiently energetic photons to bring about useful current densities, where “activation” was done by a UV laser having 257 nm wavelength to introduce color center, with energy states inside the band gap.
What is needed is a device and method of activating color centers that obtains photoelectron emission with longer wavelengths and can achieve heightened quantum efficiencies and extended photocathode lifetimes.